This application claims the priority benefit of Taiwan application serial no. 89104493, filed Mar. 13, 2000.
1. Field of Invention
The present invention relates to a synchronous switching device for fluorescent lamp. More particularly, the present invention relates to a synchronous switching device for controlling the lighting of a planar fluorescent lamp at the back of a large area liquid crystal display (LCD).
2. Description of Related Art
Liquid crystal display (LCD) is a high image quality, small volume, lightweight, low voltage driven and low power consumption image-displaying device. Nowadays, liquid crystal display (LCD) panels are widely adopted in portable televisions, mobile telephones, camera recorders, notebook computers, desktop computers as well as projectors. In fact, the conventional cathode ray tube (CRT) is gradually being replaced by LCD as the mainstream display device. However, liquid crystal display differs from other display devices such as plasma display panels (PDP), electro-luminescent (EL) displays, and light-emitting diodes (LED) in that the panel does not emit light. An external light source must be available to illuminate the LCD panel. Hence, most LCD panel needs to include a back light at the back of the panel.
Typically, the back light of a LCD panel is a linear fluorescent tube having a diameter between 1.8 mm to 2.6 mm. The linear fluorescent tube has two electrodes, one at each end of the tube, with fluorescent powder coated on the interior sidewall. The interior space of the fluorescent tube is filled with mercury vapor and inert gas. The fluorescent lamp operates by applying a voltage across the two electrodes, thereby leading to an electrode discharge and the production of plasma that emits ultraviolet radiation. When the fluorescent powder coated on the interior sidewalls of the fluorescent tube is activated by ultraviolet radiation, visible light is produced.
As area of a LCD panel increases, a uniformly lit panel capable of serving as a back lighting source is more urgently needed. Since linear fluorescent tubes are incapable of providing planar illumination, special planar fluorescent lamps have been designed. FIGS. 1A and 1B are the respective front view and the cross-sectional top view of a conventional planar fluorescent lamp. As shown in FIGS. 1A and 1B, the planar fluorescent lamp 100 includes surface panels 102a and 102b that are parallel to each other. Side panels 102c are inserted between and near the edges of the panels 102a and 102b. The surface panels 102a, 102b and the side panels 102c together form a fluorescent tube 102 enclosing a hollow space 104. Fluorescent layers 106 are formed on the interior sidewalls of the surface panels 102a and 102b, respectively. A first electrode 108a and a second electrode 108b are installed inside the hollow space 104 close to each side panel 102c. 
As surface area of the planar fluorescent lamp 100 increases, overall length of the first electrode 108a and the second electrode 108b must increase correspondingly. Due to their increased length, a voltage applied to the two electrodes 108a and 108b may not simultaneously equalize to the same potential level at every point along the entire length instantaneously. Since electric discharge follows the smallest impedance line type route between the electrodes, a line type discharge similar to a linear fluorescent lamp is produced. Hence, the intended uniform fluorescent panel for lighting a large LCD panel is not actually produced.
FIG. 2 is a front view showing the internal structure of another conventional planar fluorescent lamp. The planar fluorescent lamp in FIG. 2 is very similar to the one shown in FIGS. 1A and 1B; hence identical parts are label with the same numerals. The planar fluorescent lamp 200 has sidewall panels 102c inserted between the surface panels 102a and 102b near the edges. The surface panels 102a, 102b and the side panels 102c together form a hollow space 104. Fluorescent layers 106 are formed on the interior sidewalls of the surface panels 102a and 102b, respectively. An equal number of first electrodes 208a and second electrodes 208b are installed inside the hollow space 104 close to each side panel 102c. 
To operate the planar fluorescent lamp shown in FIG. 2, each pair of first electrode 208a and second electrode 208b must be applied synchronous identical voltage so that each pair of electrodes inside the fluorescent tube 102 emits light concurrently. Otherwise, only one pair of electrodes will carry out an electrical discharge similar to the discharge of a linear fluorescent tube. In addition, when an external voltage is applied to various electrode pairs, electric arcs may form between adjacent electrodes leading to mutual interference.
Sequential timing control can be applied to the electrode pairs inside the planar fluorescent lamp so that each electrode pair discharge in turn similar to the horizontal scanning of a cathode ray tube. However, overall brightness attained by the planar fluorescent lamp is greatly reduced compared with a planar fluorescent lamp formed by joining a series of parallel linear fluorescent tubes.
Nevertheless, although a series of fluorescent tubes on a planar panel is capable of having a higher brightness level, a diffusion panel must be inserted between the fluorescent panel and the LCD panel to equalize brightness level across the panel. Moreover, when the planar fluorescent lamp is too close to the LCD panel, layout of the fluorescent tubes inside the lamp may appear on the LCD panel, thereby affecting image quality. On the other hand, if the distance between the planar fluorescent lamp and the LCD panel is increased, overall thickness of the LCD panel and the fluorescent lamp will increase thereby adding weight and volume to the panel.
Accordingly, one object of the present invention is to provide a synchronous switching device for lighting the planar fluorescent lamp at the back of a large surface liquid crystal display (LCD) panel. A voltage signal is transmitted to the synchronous switching device. The switching device generates synchronous voltage signals and which then pass to a plurality of voltage converters where the voltage signals are amplified. The amplified synchronous signal is fed to the common electrode and a plurality of distributed electrodes inside the planar fluorescent lamp so that the lamp is switched on synchronously and arcing between electrodes is suppressed. Hence, the entire planar fluorescent lamp lights up to obtain a high brightness level and uniform light source.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a synchronous switching device. The switching device includes a common electrode, a plurality of distributed electrodes, a plurality of voltage converters and a signal generator. The distributed electrodes are aligned on a straight line facing the common electrode. Each the distributed electrode has a length smaller than the common electrode. Each voltage converter is electrically coupled to the common electrode as well as one and only one distributed electrode. All voltage converters are electrically coupled to signal generator.
According to a second embodiment, this invention provides a planar fluorescent lamp. The planar fluorescent lamp includes a first panel, a second panel, two side panels, two fluorescent layers, a common electrode, a plurality of distributed electrodes, a plurality of voltage converters and a signal generator. The first and the second panel are parallel to each other with side panels between the first and the second panel near the edges so that a planar lamp enclosing a hollow tube is produced. The fluorescent layer is deposited on the interior surface of the first and second panel, respectively. The distributed electrodes and the common electrode are mounted on each side inside the hollow space of the planar lamp adjacent to the side panels. The distributed electrodes are aligned on a straight line facing the common electrode. Each distributed electrode has a length smaller than the common electrode. Each voltage converter is electrically coupled to the common electrode as well as one and only one distributed electrode. All voltage converters are electrically coupled to the signal generator.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.